A number of atmospheric pollutants and greenhouse gases have strong fundamental vibrational transitions within the spectral range of 7.5-8 µm, which marks the region as particularly important for trace gas sensing. Here, we report the development of a mid-infrared continuouswave (cw) cavity ring-down spectroscopy (CRDS) technique coupled with an external-cavity (EC) mode-hop-free quantum cascade laser (QCL) operating at 7.5 µm. We validated the EC-QCL based high-resolution cw-CRDS system by measuring 12 CH 4 and 13 CH 4 isotopes of methane (CH 4 ) which served as a benchmark molecule. The direct, quantitative and selective measurements of 12 C and 13 C isotopes of CH 4 in ambient air as well as in human breath samples in the levels of parts per billion by volume were made by probing one of the strongest fundamental vibrational transitions of CH 4 arising from the asymmetric bending (ν 4 band) vibrations of the bonds centred at ~1327.244 cm −1 and ~1332.946 cm −1 , respectively. We achieved a noise-equivalent absorption coefficient of 1.86 × 10 −9 cm −1 Hz −1/2 with 100 Hz data acquisition rate for the current cw-CRDS spectrometer. The current high-resolution cw-CRDS system could be further exploited to harness the full advantage of the spectral region covering 7.5-8 µm to monitor several other trace molecular species along with their isotopic compositions.

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The underlying mechanisms of the triple-oxygen (16O, 17O, and 18O) isotopic content of deuterated (D) isotopologues of water in H–D exchange reactions in the gas phase remain elusive. Herein, we have demonstrated a high-resolution gas-phase spectral analysis of doubly (D2O) and singly (HDO) deuterated isotopologues of water in the region around 7.8 μm using quantum cascade laser-based cavity ring-down spectroscopy. Isotopic fractionations among doubly and singly deuterated species of water, D2 16O, HD16O, HD17O, and HD18O, in the gas phase were carried out by probing the fundamental and hot band transitions in the ν2 (bending) mode of D2O and the fundamental ν2 transitions for the other water isotopes. We subsequently investigated the fractionations of different D-enriched water isotopologues for the H–D exchange reaction using various mixtures of D2O in H2O. We explored the potential role of triple-oxygen isotopic contents through enrichments and depletions of HD16O, HD17O, and HD18O, involved in the H–D reaction. Our first clear, direct, and quantitative experimental evidence reveals a new picture of gas-phase isotopic fractionation chemistry in a mixture of light and heavy water (H2O–D2O).

A laser based N2O sensor for atmospheric measurement.

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